The present invention relates to a method of making coatable sheets having light-modulating material.
Currently, information is displayed on sheets using permanent inks or displayed on electronically modulated surfaces such as cathode ray displays or liquid crystal displays. Other sheet materials can carry magnetically writable areas to carry ticketing or financial information, however magnetically written data is not visible.
World patent application PCT/WO 97/04398, entitled xe2x80x9cElectronic Book With Multiple Display Pagesxe2x80x9d, is a thorough recitation of the art of thin, electronically written sheet display technologies. Disclosed is the assembling of multiple display sheets that are bound into a xe2x80x9cbookxe2x80x9d, each sheet provided with means to individually address each page. The patent recites prior art in forming thin, electronically written pages, including flexible sheets, image modulating material formed from a bi-stable liquid crystal system, and thin metallic conductor lines on each page.
Heretofore, U.S. Pat. No. 3,697,297, discloses material suitable for such a device. A cholesteric liquid crystal material is encapsulated by light penetrable gelatin and gum arabic capsules that are coated on a screen. The screen changes color when receiving sufficient heat energy to clear the cholesteric material.
Fabrication of flexible, electronically written display sheets using liquid crystals materials was disclosed in U.S. Pat. No. 4,435,047. A first sheet has transparent ITO conductive areas and a second sheet has electrically conductive inks printed on display areas. The sheets can be thin glass, but preferably have been formed of Mylar polyester. A dispersion of liquid crystal material in a binder is coated on the first sheet, and the second sheet is bonded to the liquid crystal material. Electrical potential is applied to opposing conductive areas to operate on the liquid crystal material and expose display areas. The display uses nematic liquid crystal materials, which ceases to present an image when de-energized. Currently, privacy windows are created using the scattering properties of conventional nematic liquid crystals. Such materials require continuous electrical drive to remain transparent.
U.S. Pat. No. 5,437,811 discloses a light-modulating cell having a polymerically dispersed chiral nematic liquid crystal. The chiral nematic liquid crystal has the property of being driven between a planar state reflecting a specific visible wavelength of light and a light scattering focal-conic state. Said structure has the capacity of maintaining one of the given states in the absence of an electric field.
The prior art discloses methods for forming polymer beads from polymeric precursors in aqueous suspension such as, U.S. Pat. No. 2,932,629. U.S. Pat. No. 2,932,629 disclose a limited coalescent method for forming spheroid particles of highly uniform size through the use of colloidal particles to limit coalescence of smaller droplets into larger, uniform domains. The polymerizable liquid is brought to given size, and a catalytic agent performs the polymerization reaction to form solid polymeric bodies having substantially uniform size. The technique of using limited coalescence for uniform bead size during polymerization is further disclosed in U.S. Pat. Nos. 3,933,771, 4,324,932, and 4,833,060.
Accordingly, it is an object of the present invention to provide a method of making a machine coatable polymerically dispersed light-modulating material of uniform domain size.
Another object of the present invention is to disperse liquid crystal material using limited coalesence and coat the dispersion to form a dispersed light-modulating layer having improved optical properties.
Sheets made in accordance with the present invention can be used to provide a re-writable image sheet. The present invention uses a colloidal solid particle emulsifier to limit domain growth from a highly dispersed state. Uniformly sized liquid crystal domains are created and machine coated to manufacture light-modulating, electrically responsive sheets with improved optical efficiency. The sheet can be formed using inexpensive, efficient photographic layer methods. A single large area of sheet material can be coated and formed into various types of sheets and cards. Displays in the form of sheets in accordance with the present invention are inexpensive, simple and fabricated using low-cost processes.
These objects are achieved by a method of making a light-modulating, electrically responsive sheet comprising:
(a) providing a substrate;
(b) forming an electrically conductive layer over the substrate;
(c) providing in an aqueous bath an immiscible, field responsive light-modulating material along with a quantity of colloidal particles wherein the colloidal particles limit coalesced domain size and a binder;
(d) blending the constituents of the aqueous bath to form a dispersion of said field-responsive, light-modulating material below a coalescence size which sets to form limited coalescence domains having a uniform size;
(e) coating said material over the substrate; and
(f) drying said coated material to form a set of uniform domains so that such dispersion coalesces to form a set of uniform limited coalescence domains having a plurality of electrically responsive optical states.
Flexible sheets can efficiently be made in accordance with the present invention that has a light-modulating layer that has domains with improved optical properties. By changing the field applied across the layer, information can be written in the sheet.